Add metrics configuration.

diff --git a/doc/markdown/configuration/metrics.md b/doc/markdown/configuration/metrics.md
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--- a/doc/markdown/configuration/metrics.md
+++ b/doc/markdown/configuration/metrics.md
@@ -12,3 +12,101 @@
 * [Modules](../modules/index.md)
 
 ## Introduction
+
+Metrics section provides configuration for symbols weights and actions apllied for a message by rspamd.
+You could imagine metric as a decision used by rspamd for a specific message by a set of
+rules. Each rule can insert a `symbol` to the metric, that means that this rule is true
+for this message. Each symbol can have a floating point value called `weight` that means
+the significance of the correspoinding rule. Rules with positive weight icrease `spam` like
+property whilst rules with negative weight increase `ham` like property. The final estimation
+is the messages's score.
+
+After a score is evaluated, rspamd selects an appropriate `action` for a message. Action
+means the desired operation that should be applied for a message based on the specific
+metric. Rspamd defines the following actions ordered by `spam like` property in ascending
+order:
+
+1. `no action` - a message is likely ham
+2. `greylist` - a message should be greylisted to ensure sender's validity
+3. `add header` - add the specific `spam` header indicating that a message is likely spam
+4. `rewrite subject` - add spam subject to a message
+5. `soft reject` - temporary reject a message
+6. `reject` - permamently reject a message
+
+Actions are assumed to be applied simultaneously, meaning that `add header` action implies,
+for example, `greylist` action. `add header` and `rewrite subject` have actually the same
+power in terms of rspamd. They are just two options of the same purpose: to mark a message
+as probable spam. `soft reject` action is mainly used to indicate temporary issues in mail
+delivery, for instance, rate limit reaching.
+
+There is also special purpose metric called as `default` that acts as the main metric
+to treat a message as spam or ham. Actually, all clients that use rspamd just check the
+default metric to determine whether a message is spam or ham. Therefore, the distribution
+configuration defines merely the `default` metric.
+
+## Configuring metrics
+Each metric is defined by a `metric` object in rspamd configuration. This object has one
+mandatory attribute - `name` which defines the name of this metric:
+
+~~~nginx
+metric {
+   # Define default metric
+   name = "default";
+}
+~~~
+It is also possible to define some generic attributes for the metric:
+
+* `grow_factor` - the multiplicator applied for the subsequent symbols inserting by the follwing rule:
+
+       score = score + grow_factor * symbol_weight
+       grow_factor = grow_factor * grow_factor
+
+by default this value is `1.0` meaning that no weight growing is defined. By increasing this value you
+increase the efficient score of messages with multiple `spam` rules matched. This value
+is not affected by negative score values.
+
+* `subject` - string value that is prepended to the messages's subject if `rewrite subject`
+action is applied
+* `unknown_weight` - weight for unknown rules. If this parameter is specified, then all rules can
+insert symbols to this metric. If such a rule is not specified by this metric then its weight is equal
+to this option's value. Please note, that adding this option means that all rules will be checked by rspamd, on the
+contrary, if no `unknown_weight` metric is specified then rules that are not registered anywhere are silently ignored
+by rspamd.
+
+
+The content of this section is separated to the two main parts: symbols and actions.
+Actions section is an object of all actions defined by this metric. If some actions are skipped,
+they won't be ever suggested by rspamd. Actions section looks as following:
+
+~~~nginx
+metric {
+...
+       actions {
+               reject = 15;
+               add_header = 6;
+               greylist = 4;
+       };
+...
+}
+~~~
+
+You can use `_` symbol instead of space in action names to simplify the configuration.
+
+Symbols are defined by an object with the following properties:
+
+* `weight` - the symbol weight as floating point number (negative or positive), by default the weight is `1.0`
+* `name` - symbolic name for a symbol (mandatory attribute)
+* `group` - a group of symbol, for example `DNSBL symbols` (as shown in webui)
+* `description` - optional symbolic description for webui
+
+So far, the symbol definition looks like this one:
+
+~~~nginx
+symbol { 
+    name = "RWL_SPAMHAUS_WL_IND"; 
+    weight = -0.7; 
+    description = "Sender listed at Spamhaus whitelist"; 
+}
+~~~
+
+A single metric can contain multiple symbols definitions.
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